1. Field of the Invention
The present invention relates to an apparatus for detecting a variation of electrostatic capacitance. More specifically, the present invention relates to an improvement in an apparatus for detecting a minor variation of electrostatic capacitance occurring between a stylus and a disc in a video disc apparatus of the CED type or the VHD type, or in a measuring apparatus of any various types, for example.
2. Description of the Prior Art
A video disc apparatus for reproducing sound and video signals recorded on a disc has been proposed. Such video disc apparatus has been implemented as a CED (Capacitance Electronic Disc) type or a VHD (Video High density Disc) type. In such a video disc apparatus, reproduction is made of information upon detection of a variation of electrostatic capacitance between a stylus and a disc on which information is recorded. However, a variation of the capacitance between a stylus and a disc is as small as C.apprxeq.10.sup.-4 pF and therefore an FM video signal recorded on the disc can hardly by directly read from the variation of the electrostatic capacitance. Therefore, a resonance circuit having a resonance frequency sufficiently higher than the maximum frequency of the FM video signal is provided so that a capacitance variation from the stylus is transferred to the resonance circuit to change the resonance frequency thereof. An oscillation signal produced by an oscillation circuit having an oscillation frequency sufficiently larger than the maximum frequency of the FM video signal is applied to the above described resonance circuit so that amplitude modulation is made by passing the oscillation signal through the resonance circuit, the resonance frequency of which varies in accordance with the variation of the capacitance between the stylus and the disc. The FM video signal can be read using an envelope detector which detects the envelope of the amplitude modulated signal.
FIG. 1 is a schematic diagram of a conventional FM video signal reproducing circuit which is employed in a video disc apparatus of either the CED type or the VHD type in which the present invention can be employed. Referring to FIG. 1, a terminal 1 coupled to a pick-up, i.e. a stylus, (not shown) is connected through a coil 2 and a capacitor 3 to ground. Accordingly, the coil 2 and the capacitor 3 together with the electrostatic capacitance between the stylus and the disc constitute an LC parallel resonance circuit. The parallel resonance circuit has a resonance frequency of, for example, 910 MHz. In practice, a stripline is formed on a board, not shown, of fluoride resin such as Teflon (trademark) and the fluoride resin board is covered with a metallic casing. More specifically, a coaxial resonator is formed with the above described stripline, the fluoride resin board and the metallic casing. A metalic electrode is provided at the tip end of the stylus, while the disc is made of a conductive material and is connected to the ground. Therefore, an electrostatic capacitance value between the stylus and the disc changes as a function of grooves or pits formed on the disc. Accordingly, this structure is equivalent to a parallel connection of a capacitor of a capacitance value which changes in accordance with the groove or the pit to the above described coaxial resonator, wherein the resonance frequency of the resonator fluctuates with respect to the central frequency of 910 MHz.
Furthermore, an oscillator 4 oscillating at the frequency of, for example, 915 MHz, a detector 5, and an amplifier 6 for amplifying the output from the detector 5 are provided on the above described fluoride resin board. The oscillator 4 and the coaxial resonator are electromagnetically coupled or antenna coupled and similarly the coaxial resonator and the detector 5 are also electromagnetically coupled. More specifically, coupling striplines 7 and 8 are provided on the board in the vicinity of the stripline constituting a portion of the resonator. The oscillation output from the oscillator 4 is supplied to the stripline 7 and the stripline 8 is connected to the detector 5. Accordingly, the signal of the frequency of 915 MHz transmitted from one stripline 7 is amplitude modulated by the resonator in accordance with the variation of the capacitance between the stylus and the disc and is received by the other stripline 8. The amplitude modulated signal received by the stripline 8 is envelope detected by the detector 5 and is converted into an FM video signal. The FM video signal is amplified by the amplifier 6 and is withdrawn from the output terminals 9. The purpose of employing such electromagnetic coupling or antenna coupling is to achieve loose coupling so as to attain as large a quality factor as possible inasmuch as the conventional circuit configuration has a small quality factor.
Although the above described circuit employs expansive fluoride resin for the circuit board in order to treat the high frequency signal efficiently, as a practical matter it is not possible to attain a sufficient quality factor and the stability of the oscillator is poor even with such a circuit board. Since the above described circuit employs a coaxial resonator having a stripline formed on a circuit board and cooperating with a metallic casing, the circuit becomes bulky. In addition, the dielectric constant of fluoride resin employed in the circuit board is not large and hence there is a limit to a degree of miniaturization attainable. Furtheremore, the linear expansion coefficient of the board is large and hence the dielectric constant of the board is changeable depending on a change in temperature, which gives rise to frequency fluctuation. Thus, a conventional circuit has a poor temperature stability of the frequency and hence generally requires an automatic frequency control. Therefore, a circuit configuration becomes complicated and bulky and at the same time expensive.